A color cathode ray tube having an aperture grill is shown in FIG. 7.
Aperture grill 1 has an array of stripe-shaped slits 2 oriented in a vertical direction which are made by etching a thin sheet of metal. A metal strip located between two adjacent slits 2 is called grill tape 2a. A frame 3 is made by welding and unifying an upper part 4, a lower part 5, a right part 6 and a left part 7. The upper and lower ends of the aperture grill 1 are respectively welded to the upper part 4 and the lower part 5 of the frame so that each grill tape 2a is stretched between the upper part 4 and the lower part 5 with a predetermined tension.
A damper line 8 is stretched between two leaf springs 9 respectively attached to the right part 6 and the left part 7 along the surface of the aperture grill 1. The damper line 8 prevents a grill tape 2a of the aperture grill 1 from vibrating mechanically. A color cathode ray tube of 14-16 inch typically has a single damper line. A color cathode ray tube of 18-25 inch typically has two damper lines. A color cathode ray tube being bigger than 25 inch typically has three damper lines. Thus, the number of a damper line attached to a color cathode ray tube depends on the size of a color cathode ray tube.
FIG. 8 is a cross-sectional view of a fluorescent stripe and a carbon stripe. FIG. 8 shows an aperture grill 1, a slit 2, a grill tape 2a, a glass panel 10, a fluorescent stripe 11 and a carbon stripe 12 located between two adjacent fluorescent stripes. The fluorescent stripe 11 is repeatedly arranged in the order of red, green and blue stripes.
A white ratio defined by the formula (1) is approximately between 50% and 60% on a color cathode ray tube having an aperture grill. EQU WHITE RATIO [%]=3W.sub.PS /P.sub.SC *100 (1)
Here, W.sub.PS is the width of a fluorescent stripe. P.sub.SC is the length of from a red fluorescent stripe to the next red fluorescent stripe.
An electron beam 13 is emitted from an electron gun (not shown in figure). An electron beam 13 irradiates fluorescent stripes 11 on a glass panel 10 through a slit 2 of an aperture grill 1. As an example, the electron beam 13 of FIG. 8 irradiates a green fluorescent stripe.
A color cathode ray tube having an aperture grill has advantages over other types of color cathode ray tubes not having an aperture grill. However an aperture grill type color cathode ray tube has a disadvantage that a grill tape easily vibrates, for example, by a voice output from a speaker provided with a cathode ray tube in a television receiver or computer monitor because the width of a grill tape is very small. Such vibration can deteriorate the picture quality.
For instance, a 20 inch color cathode ray tube inch has a grill tape of length 286 mm, thickness 0.13 mm, and width 0.22 mm. A grill tape is regarded as a string relating to the vibration of a grill tape. Grill tape 2a moves in both the X axis and the Z axis directions as shown in FIG. 8. When a grill tape moves in the direction of the X axis, a misalignment 51 is created between the center of an electron beam 52 and the center of a fluorescent stripe 53 receiving the electron beam thereon. Thereby, the fluorescent stripe 11 has an area 55 where no light is emitted. The light emitting area of the fluorescent stripe 11 is shown as the number 54 in FIG. 8. When there is no discrepancy between the center of an electron beam 52 and the center of a fluorescent stripe 53 receiving the electron beam thereon, the entire fluorescent stripe 11 emits light.
The width of the area 55 in which no light is emitted is changed by the vibration of a grill tape 2a. Therefore, the quality of light emitting from the fluorescent stripe 11 is changed as shown in FIG. 9(A) and 9(B). FIG. 9(A) shows the quality of light changing over a very short time period. FIG. 9(B) shows a quality of light changing in a time period wherein the picture quality is deteriorated. The time period of FIG. 9(B) is comparatively longer than the time period shown in FIG. 9(A). The picture quality deteriorating time period is a time period when the picture is distorted by vibration of a grill tape 2a. The picture quality deteriorating time period includes a time period effected by vibration even after the cause of the vibration is removed. For example, a picture quality deteriorating time period might be on the order of 0.7 seconds.
The notations "RI", "L1" and "R2" of FIG. 9(A) each indicate a period of time wherein either a right (R) or left (L) side of the fluorescent stripe 11 is emitting no light due to misalignment of the strip 11 with the slit 2. For example, during time R1, fluorescent stripe 11 has an area 55 with no light emitted at the right side (R1). During time L1, stripe 11 has an area 55 on the left side wherein no light is emitted. The misalignment is maximum and thus the area 55 is maximum at about the center of time period R1, L1, R2, L2, etc., and thus the amount of light emitted from the strip 11 is lowest at the center of each of these time periods. Between these areas where the strip 11 is misaligned with the grill slit 2, the strip 11 is fully illuminated by beam 13 and thus emits maximum light.
The change of the quantity of light generated by area 55 attenuates as shown in FIG. 9(B) as time goes by, due to the attenuation of vibration as time goes by. When the vibration stops, the change of the quantity of light becomes zero.
The period time when a picture is affected by vibration of the grill tape 2a is called "picture quality deteriorating time period" shown as 60 in FIG. 9(B). The picture quality deteriorating time period 60 includes a time period caused by the vibration of the aperture grill which persists even after the cause of the vibration is removed. It is desired to shorten the picture quality deteriorating time period 60, for example to 0.5 seconds or less. This is especially the case for color cathode ray tubes used for a computer display. Since computer displays require very high picture quality, even small deterioration of picture quality is undesirable. Further, because a monitor for a computer display is now commonly used with a speaker, the picture deterioration generated by the speaker can not be ignored.